This invention relates to an evaporation boat, and more particularly to an evaporation boat for evaporating material to form a thin film of the material on a substrate by vacuum deposition technique wherein there is a tendency of depositing particles larger than 1 micron.
Vacuum deposition technique, for example, is used to form a photoconductive film, optical stripe filters, dichroic mirrors or the like which constitute a tanget for a pick up tube.
As the evaporation sources for vacuum deposition are used a crucible, coil or boat depending upon the field of application. Among these various types of the evaporation source, evaporation boats are used most widely because of its cheapness and easiness in handling. Most of the boats have a simple construction utilizing a thin sheet made of a metal having a high melting point, tungsten, molybdenum and tantalum, for example, and provided with a recess. The longitudinal opposite ends of the boat are connected to an electric source to heat and evaporate vapourizable material contained in the recess.
With a boat of such simple construction when the material to be evaporated comprises a sublimatable material, solid particles having a diameter of several microns also formed together with the vapour of the material.
When such solid particles having a diameter of larger than 1 micron are contained in the vapour deposited films, the quality thereof would be greatly degraded. For example, the light receiving area of SATICON tube (trade mark) is 12.7 mm .times. 9.5 mm for a tube diameter of 1 inch, and 8.8 mm .times. 6.5 mm for a tube diameter of 2/3 inch. In an image monitor, an enlarged image, for example 14 .times. 21 inches, is formed on an image receiving tube. Accordingly, should the photoconductive film of an image pick up tube contain massive foreign matters, picture defects in the form of dark spots or bright spots, reproduced on the screen of an image receiving tube, would be formed thus greatly degrading the quality of the picture.
To eliminate this defect, a so-called laminated type evaporation boat as shown in FIG. 1 has been proposed. As shown in FIGS. 1A and 1B, the boat 11 comprises upper and lower recessed sheets 14 and 12 and an intermediate partition sheet 13. The intermediate partition sheet 13 is provided with two spaced perforations 13a while the upper sheet a single perforation 14a at a point intermediate of said perforations 13a. The material to be evaporated 15 is contained in the recess of the lower sheet 13. When current is passed through the boat by connecting it across a source of supply the material 15 is evaporated by the heat of the boat and the vapour of the material is ejected through tortuous passages including perforations 13a and 14a. Solid particles having a diameter larger than 1 micron are removed by the collision thereof against the inner wall of the upper sheet 14.
With this laminated type evaporation boat as the vapour much pass twice through perforations thus decreasing the vapour deposition speed. In addition, this construction requires higher temperature for the boat.